He wants the readers of RealClimate.org to dismiss the importance of the observation that the climate has always changed. In order to achieve this goal, he fights four straw men – he chooses four propositions that he believes not to follow from the proposition "the climate has always changed", even though at least some of them actually do follow, as I will discuss.

Too bad. If he had at least some integrity, he would study the actual reasons why people point out that the climate has always changed instead of his straw men – or I should perhaps say straw persons because he's a writer for a far left-wing activist website.

OK, why do many of us emphasize that the climate has always changed? It's because this important proposition of ours is primarily a response to statements such as

Climate is changing.
Climate change is real.

Haven't you heard these sentences? I have heard them thousands of times. You get a million of hits at Google even if you look for the precise form of these sentences.

We say that the climate has always changed in order to point out that those who say "climate is changing" or "climate change is real" don't have the slightest clue about this scientific discipline. The sentences such as "climate is changing" are formally true but they also implicitly say that

It is important information and a modern scientific discovery that climate is changing.
Or: A brand new process has started in the world we inhabit.

But these sentences are absolutely untrue because the climate has always changed. Everyone who says "climate is changing" and indicates that it is some important information that should affect our behavior if not policies influencing the whole national economies is a scientifically illiterate, probably brainwashed, layman or an intentionally deceptive demagogue or a liar.

The climate has always changed so the sentences such as "climate is changing" are as worthless tautologies as 2+2=4. It it silly to repeat such a statement and it is absolutely silly to suggest that such a sentence is a deep or original insight that may support one policy or another.

These comments of mine were qualitative. Once you look beneath the surface, you want to know how much the climate has changed in the past, how many problems those changes caused, how much the climate has changed recently, what is the ratio of the typical trends, whether the recent changes may be mostly assigned to the human activity, and whether the expected changes in the future may be dangerous in some way.

Throughout the geological history, the climate has changed due to many factors – changing output of the Sun, changing chemistry of the atmosphere (while it is over 20% now, oxygen wasn't always there), the continental drift, probably the motion of the Solar System through the arms of the Milky Way, and many other reasons.

Some quantities that describe important characteristics of the climate were changing a lot. Richard Lindzen likes to say that the pole-to-equator temperature difference is the most important parameter of this kind. It has varied between 20 and 60 °C and now is closer to 40 °C, somewhere in between. These changes were dramatic, of order 100%. This temperature difference was arguably more important for the dynamics of the Earth's atmosphere than the currently overhyped global mean temperature.

But even the global mean temperature has been changing a lot. Even if you average the global mean temperature over very long, 1-million-year-long intervals – which should arguably average out the differences and make the graph stable – the temperature has been changing in a window whose width is comparable to 10 °C:

The graph covers the Earth's history from its birth 4.6 billion years ago – the \(x\)-axis isn't uniform. Even if you look at recent 450,000 years,

you will notice the glaciation cycles. Ice ages were some 8 °C cooler than the interglacials. And it takes just a few dozens of thousands of years for the climate to change this much. Be sure that the human race was already around when the climate was switching from a cooler one to a warmer one – by 8 °C or so – many times. Relatively to the changes I discussed, the observed change of the global mean temperature since the Industrial Revolution is basically negligible (it's still below 1 °C) so even if 100% of the 20th century climate change were due to humans, there is absolutely no reason for a worry.

Even though the influence of the mankind on the climate is nonzero – the influence of almost anything on almost anything else is nonzero in principle so this shouldn't be shocking – there doesn't exist the slightest reason to be concerned about the ongoing or recent climate change. It has clearly caused no problems whatsoever. It is a waste of time to even talk about the climate worries – it's something that only very stupid people or those who make a profit out of the very stupid people may repeatedly discuss.

But let me discuss Rahmstorf's four straw men. Rahmstorf tells us that "the climate has always changed" doesn't imply either of the following four propositions:

(1) that humans cannot change the climate
(2) that we do not know whether humans are to blame for global warming
(3) that global warming will not have any severe consequences
(4) that we cannot stop global warming

Is it true that none of these four propositions is "implied" by the proposition "the climate has always changed"? Well, it depends on the rigor you assume behind the verb "imply". If you demand some rigorous mathematical proof that is absolutely free of any conceivable loopholes, then indeed, you can't write down either of the four rigorous proofs. But it's true in natural science in general: almost no implications or other statements can be proven quite rigorously. Natural science isn't just mathematics. Natural science cannot be quite rigorous according to the mathematician's taste.

But if you talk about implications that could be relevant for scientifically well-informed decisions, about the common understanding of the implication, and if you're satisfied with the probability higher than 99% as the equivalent of certainty, things are different. So let's ask whether the "facts normally associated with the statement that the climate has always changed" almost reliably or at least morally imply "the facts associated with the four propositions above". Well, they mostly do.

First, does "the climate has always changed" imply that "humans cannot change the climate"? Strictly speaking no. But one should be careful about the actual meaning that is being assigned to the sentence "humans change the climate". The sentence could mean "the humans' influence on the climate is strictly speaking nonzero". Such a refined proposition is true. But because everything influences everything else – the butterfly wings in Beijing influence the weather in New York for the next month, champions of chaos theory love to say – such an interpretation of the sentence would be inconsequential. And it's not what the people actually mean by "humans change the climate".

Instead, what they mean is that "humans change the climate in some unprecedented, qualitative way, they determine some basic character of it". But once you adopt that this is what is meant, the proposition "the climate has always changed" does imply that the "humans aren't changing some basic qualitative character of the climate" etc. Why? Because the correct statement that "the climate has always changed" includes the true insight that the constant and unstoppable changes are an inseparable part of the very concept of the climate. And the changes continue and are going to continue – so the humans haven't changed anything about them. Equivalently, you may say that the implication holds if you use the verb "determine": the humans can't determine the climate (reliably determine, or determine as the only factors) because for billions of years, it was determined by natural processes that didn't disappear. At most, humans may have several "votes" that decide about certain questions but they can't be the "rulers" of the climate because Nature hasn't escaped the Earth yet.

At the same moment, the known numbers – even the overstated numbers promoted by the IPCC – imply that the human influence is smaller than the natural changes that have always taken place. The climate sensitivity is probably comparable to 1 °C per doubling but even if it were 3 °C, above the most likely IPCC value, the expected temperature change from the burning of all fossil fuels would still be smaller than the changes that were ignited by the glaciation cycles and other sources.

If I (or someone else) were treating the sentences sensibly, I really should understand these basic numbers to be a part of the observation that "the climate has always changed". And once I do, the proposition does imply that the humans aren't making an important, game-changing impact on the climate.

Now, does the sentence "the climate has always changed" imply that "we do not know whether humans are to blame for global warming"? Again, it depends what you mean by the childish phrase that "humans are to blame". First, science isn't a "blame game" – it doesn't divide phenomena to "nice" and "evil" ones – so if someone seriously uses this language, you may be sure that he is not a serious scientist.

But even if you decided to tolerate this outrageously manipulative unscientific language chosen by Mr Rahmstorf, and you certainly shouldn't tolerate it, the phrase about "blaming" would still be totally ambiguous. How much influence is enough for a "blame"? Again, the influence is nonzero but it is small. Is it enough to "blame" humans? Well, the U.S. media "blame" Putin for Trump's victory because he dares to have the same nationality as a female lawyer who shook Donald Trump Jr's hand. So if you wish, you may blame anybody for anything. But such a "blame game" isn't necessarily compatible with your being a sane, honest person in the eyes of others. Just like in the previous point, what the climate fearmongers actually want the listeners to "hear" behind this sentence is that "humans are a necessary condition for some terrible, profound, qualitative change of the climate".

But we know it's not true. The very fact that the climate has always changed means that the continuing change of the climate supports the proposition that the humans haven't changed a damn thing, at least not about the big or qualitative characteristics of the climate.

Third, does the sentence "the climate has always changed" imply that "global warming will not have any severe consequences"? Yes, of course, the first sentence does imply the second one, according to the natural scientist's understanding of the implication. The Earth has gone through huge chemistry changes, asteroid strikes, mass extinctions caused by them and other events, and lots of other things. We know that none of these things were dangerous for the survival of life on the planet and only the self-evidently big changes had a chance to lead to severe consequences.

We have basically 4.6 billion years of observational evidence supporting the statement that "in the absence of an asteroid strike, change of the chemistry of the atmosphere by dozens of percent, or a similarly self-evidently big change, the more minor changes never have any severe consequences". Because we are talking about changing a trace gas that represents (and represented) about 400 ppm (or 280 ppm) of the atmosphere, which is just 0.04%, the changes are self-evidently tiny. Because the empirical evidence shows that even changes greater by many orders of magnitude failed to produce "severe consequences" at the global level, at least when we talk about consequences that emerge in a few centuries or more quickly, we know that this minor change of the chemistry of the atmosphere can't lead to "severe consequences", either.

Again, the argument above could be refined and supplemented with details but it will never be a rigorous mathematical proof. It can't be one because natural sciences aren't rigorous mathematics – and to make things worse, climatology is one of the softer natural sciences. But as long as you are reasonable and pay some attention to numbers, their proportions, and rational arguments, you must know that the third implication that Mr Rahmstorf has denied is at least morally true.

If there were no severe consequences caused by any minor changes during those very long 4.6 billion years of the Earth's history, there will almost certainly not be severe consequences of our change of a trace gas in the atmosphere, either – especially because we already know that a 40% increase of this gas in the atmosphere (between 1750 and 2017) had no undesirable consequences, let alone severe ones.

The certainty of this implication is not perfect but it is approximately the same as the certainty of the statement that the Sun will rise tomorrow again. The Sun has risen for those 4.6 billion years and we know that it won't be around forever. But could it be that the Sun will refuse to operate tomorrow? Can't the concentration of hydrogen in an important layer of the Sun drop beneath a tipping point tomorrow so that the Sun will be turned off within 24 hours? We can't exclude this possibility of such a tipping point quite rigorously. In principle, academically speaking, there may be unusual tipping points and we may be unlucky enough so that they will become relevant tomorrow and the Sun will explode or be turned off. But it's just very unlikely. If things like that were possible, we would probably see some signs of this possibility around, perhaps in our observations of other stars etc.

The very fact that the Sun has worked so nicely for 4.6 billion years is a good reason to believe that it will be our servant for several additional years. The case of the "severe consequences for the Earth's climate" is completely analogous. In the absence of the asteroid-scale events, we have 4.6 billion years worth of evidence that minor events can't have severe global consequences, at least not those that would take place quickly (in centuries or faster). The Sun and the Earth will be just fine in 50 or 150 years. It is a proof of someone's scientific illiteracy if he seriously doubts the previous sentence.

The catastrophes that the fearmongers like Mr Rahmstorf are proposing are indeed totally analogous to the "Sun that will be turned off tomorrow because a tipping point is crossed". They propose all these dramatic events – it is usually unclear whether they're joking. The Antarctica's ice sheet will break down, travel to Siberia, melt, open the permafrost, the methane will stop the Gulf Stream, and the Earth will start to spin in the opposite direction, or something along these lines. We can't rigorously prove that such never-before-seen events are impossible. But every scientifically literate person knows that it is extremely unlikely that such dramatic tipping points exist at the nearby levels – it is approximately equally unlikely as the tipping point that will kill the Sun tomorrow. One becomes a superstitious paranoid believer, and not a scientist, if he becomes obsessed with similar scenarios.

Finally, Mr Rahmstorf claims that "the climate has always changed" doesn't imply that "we can't stop global warming". Again, when the propositions are understood as they are expected to be understood, this implication actually holds, too. "The climate has always changed" really means that "the climate change is an effective law of the Earth" so nothing can be changed about its validity. About 50% of the time, the climate change involves "global warming", 50% of the time, it involves "global cooling". There have always been both phases and there will always be both phases. So we can surely not stop global warming forever.

Well, again, there may be loopholes. In the future, every squared or cubic meter of the Earth or the atmosphere may be effectively or literally air-conditioned so people will be able to change the temperature, pressure, and humidity basically everywhere. But we know that these science-fiction comments have nothing to do with the economically feasible plans for a foreseeable future. Even the sheikhs in Dubai only want to make individual cities, and not the whole country, air-conditioned. ;-)

To summarize, we say "the climate has always changed" in order to point out that everything that is true about the memes that the climate fearmongers love to spread is a vacuous tautology, a trivial implication of the science that even the schoolkids should know, and none of these basic memes implies that it's wise to get panicked let alone change the regulations affecting big parts of the economy. Mr Rahmstorf says that the observation that "the climate has always changed" has no implications for wise, science-driven policymaking. But it actually has lots of profound implications.

Mr Rahmstorf and his soulmates want to sell the proposition saying that "humans started an unprecedented process on Earth, the climate change" which is completely untrue, however. They find the longevity of the Earth inconvenient, they want the people to overlook it, but this behavior of theirs effectively turns them to Young Earth Creationists. The point is that the climate has been changing for billions of years and the life on Earth – and even the basic groups of organisms and major species etc. – have totally survived billions of years (and even the humans have survived dramatic climate changes in recent 1-2 millions of years) and vastly more profound changes than the changes that the humans are causing to the atmosphere.

It's really this perspective that is aware of the history of the Earth – including all the diverse events and 4.6 billion years of life that thrived in these changes – that shows how profoundly unscientific and creationist-like the religion of the climate hysteria is.